1. Field of the Invention
The present invention relates to radio telecommunications systems, and more particularly, to radio local loop systems.
2. Brief Description of the Related Art
A radio local loop system (xe2x80x9cRLLxe2x80x9d) is a wireless telecommunications system, wherein fixed subscriber units or terminals communicate with the system over an air interface. Such radio systems are connected to private or public switched telephone networks and include a number of radio domains, each radio domain containing one or more radio stations (RS), or base stations. Each radio station controls the wireless communication links with any number of fixed subscriber units located in a corresponding geographical coverage area. A control unit for each radio domain stores and maintains a subscriber list containing the identification codes for each fixed subscriber unit assigned to that radio domain.
A fixed subscriber unit is typically either immobile or limited in its ability to be moved during operation (e.g., as is the case with a cordless telephone). All communication with the fixed subscriber unit is handled through a radio station servicing a corresponding coverage area in which the fixed subscriber unit is located. The fixed subscriber unit has a transceiver and an antenna for transmitting and receiving telecommunications data to and from the radio station via the air interface, over at least one pre-assigned radio channel, wherein a radio channel is defined by any number of different channel access schemes.
One such channel access scheme is known in the art as time division multiple access (TDMA). In a TDMA based system, such as a TDMA based RLL system, each of a number of frequency carriers is subdivided into a number of time slots. By subdividing each frequency carrier into multiple time slots, the traffic capacity of the system is substantially increased as each of a number of fixed subscriber units are able to communicate over a single frequency carrier by limiting the time during which each transmits or receives data and control information to one or more assigned time slots.
A TDMA based system may further be characterized as either a time division duplex (TDD) system or a frequency division duplex (FDD) system. In a TDMA/FDD system, each frequency carrier is subdivided into time slots as described above. However, certain frequency carriers are dedicated to carrying downlink traffic (i.e., data and/or control information being transmitted from a radio station to a fixed subscriber unit), while other frequency carriers are dedicated to carrying uplink traffic (i.e,. data and/or control information being transmitted from a fixed subscriber unit to a corresponding radio station). In contrast, each frequency carrier handles both uplink and downlink traffic in a TDMA/TDD based system, such that approximately half of the time slots associated with a given frequency carrier are predesignated for carrying downlink traffic, while the remaining time slots associated with that frequency carrier are predesignated for carrying uplink traffic. A RLL system that employs the well-known Digital Enhanced Cordless Telecommunications (DECT) standard is an example of a TDMA/TDD based system.
In recent years, the demand for wireless radio communication services, and in particular, fixed radio communication services, has increased at an extraordinary rate. This is problematic because radio network resources are generally limited, thereby limiting both the geographic area that a system is capable of covering and limiting the amount of traffic (i.e., the traffic load) that a system is capable of handling. Certainly, one way to address this problem would be to construct new networks and/or to expand existing networks; however, such a solution would be extremely expensive.
An alternative solution to these and other related problems has been to increase the maximum range (i.e., the maximum operating distance between a fixed subscriber unit and a radio station), thereby increasing coverage area, by increasing the gain factor G of the antenna associated with each of the fixed subscriber units, wherein range is generally determined by the following relationship:   RANGE  =      G          (              T        +        I            )      
and wherein T is the noise temperature at the receiver and I represents interference. The gain factor G can be increased in a number of different ways. First, the gain factor G can be increased by simply boosting transceiver power. Unfortunately, this is generally an unacceptable option because boosting power is likely to result in a corresponding increase in the interference level in the geographic coverage area, as well as neighboring geographic coverage areas. Moreover, in accordance with the relationship above, an increase in interference levels would actually have the effect of limiting range.
The gain factor G can also be increased through the use of directional antennas. Directional antennas achieve a greater gain factor G by producing a significantly more narrow antenna beam. The use of directional antennas to generally increase the coverage area of an RLL system is a more desirable option than boosting transceiver power because it does not typically lead to increased interference levels. However, there are other problems associated with the use of directional antennas. As the transmit and receive antenna beams are generally more narrow, the task of directing (i.e., steering) the antenna beams so that they are accurately pointing in the direction of the radio station is somewhat more difficult.
Presently, the use of directional antennas requires that highly trained personnel install or perform regular adjustments on fixed subscriber units to insure that the antennas are, in fact, accurately pointing toward the corresponding radio station. And, as one skilled in the art will readily appreciate, this is extremely expensive, particularly if the RLL system is constantly undergoing network reconfiguration and/or network expansion to include the addition of new radio stations. Accordingly, in a fixed RLL system, it would be desirable to have fixed subscriber units that employ directional antennas but without the need to perform complex installation and/or readjustment procedures, or the expense associated therewith.
The present invention generally relates to a method and/or communication system that involves the dynamic selection of communication channels by fixed subscriber units in a RLL system, wherein the fixed subscriber units employ directional antennas whose scan angle (i.e., the angle representing the direction in which radio frequency energy is being transmitted and received through the directional antenna) can be automatically adjusted and then selected as part of the dynamic channel selection process, and wherein the communication channel over which a fixed subscriber unit communicates is defined not only by the radio station through which it communicates, the frequency carrier over which it communicates, and the one or more time slots during which it communicates, but also by the scan angle of the directional antenna.
In accordance with one aspect of the present invention, the communication system includes a radio station and a terminal. The terminal includes an antenna through which said terminal transmits and receives signals with a radio station while varying the scan angle of the antenna. The system also has means for determining a quality factor for each signal. Then, based on the quality factor at each scan angle, the system can select a communication channel over which the terminal can communicate with said radio station.
In accordance with an additional aspect of the present invention, the communication system includes a terminal and a plurality of radio stations. The terminal has an antenna whose scan angle can be automatically varied. The terminal measures received signals from the plurality of radio stations and determines a quality factor for each communication channel defined by a frequency carrier, time slot, antenna scan angle, and/or radio station combination. Based on the signal quality factor at each antenna scan angle, the terminal selects a communication channel over which said terminal can communicate with one or more radio stations.
In accordance with a further aspect of the invention, the improved method for performing dynamic channel selection includes the steps of transmitting and receiving signals from a radio station to a terminal while varying the scan angle of an antenna associated with the terminal, determining a quality factor for each communication channel while the transmit and receive antenna scan angle is varied, and then selecting a communication channel over which said terminal can communicate with said radio station based on the signal quality factor.
The present invention provides advantages over the prior art. First, this improved dynamic channel selection scheme allows the subscriber unit to automatically select an available traffic channel that exhibits the best signal quality characteristics for establishing a connection. Second, the improved dynamic channel selection scheme can be used in a communication system using narrow beam antennas. Thus, the cost efficiency from increased radio station range will not be lost since the installation or adjustment of antennas is not limited to highly trained personnel.